Objectives Determine if culture of oocytes from nonstimulated macaque monkeys during in vitro maturation (IVM) with freshly recovered granulosa cells obtained from FSH-primed vs. nonstimulated monkeys improves meiotic and developmental capacity of oocytes and confers on them competence to develop into blastocysts in vitro. ABSTRACT:Antral follicles r1mm in diameter were dissected from excised ovaries of 11 nonstimulated rhesus monkeys. Cumulus-enclosed germinal vesicle stage oocytes (n = 282) were randomly cultured in each of three IVM treatments 1) control (no granulosa cells), 2) nonstimulated granulosa granulosa cells (5 x 106 live cells/ml) from nonstimulated monkeys, or 3) FSH-primed granulosa granulosa cells (5 x 106 live cells/ml) from monkeys primed with purified FSH for 8.5 days. All treatments contained gonadotropins (5 g/ml FSH; 10 g/ml LH). Oocytes were cultured for 36-41 h in modified CMRL-1066 medium containing 20% bovine calf serum, then inseminated and cultured in the same medium (minus gonadotropins) until developmental arrest or zona escape. Neither nuclear maturation, fertilization or cleavage to the 8-cell stage differed among treatments. Development to the 9-15-cell stage was improved (Ps0.01) by culture with either nonstimulated granulosa or FSH-primed granulosa cells compared to control. Culture of oocytes with FSH-primed granulosa, but not nonstimulated granulosa cells, enhanced (Ps0.05) development to the morula stage compared to control (23.1, 10.7 and 5.3%, respectively). Oocytes cultured with FSH-primed granulosa cells were more (Ps0.05) competent to develop into blastocysts (6.5%) than control (0.5%) or those cultured with nonstimulated granulosa cells (0.6%). Development into expanded and hatched blastocysts was also improved (Ps0.05) by culture of oocytes with FSH-primed granulosa cells vs. nonstimulated granulosa cells or control. In conclusion, culture of immature oocytes from nonstimulated monkeys with fresh granulosa cells recovered from FSH-primed monkeys enhanced their competence to develop into blastocysts and hatch in vitro, and resulted in the first blastocysts produced in vitro from IVM/in vitro fertilized (IVF) oocytes of nonstimulated primates. Keywords oocyte maturation, embryo, blastocyst, follicle-stimulating hormone priming.